1. Field of the Invention
This invention relates to a process for deprotecting chemically amplified resist materials in a controlled manner and the use of these materials in lithographic processes for device or photomask fabrication.
2. Art Background
Lithographic processes are typically employed in the manufacture of devices such as semiconductor devices, integrated optics, and photomasks. Such processes utilize various energy sources to create a relief image in a film of resist material applied onto a substrate. Energy sources that are typically used in lithographic processes include light and electron beam radiation.
Lithographic processes frequently employ resists that contain polymeric materials. Some polymeric resist materials, such as those described in U.S. Pat. No. 4,812,542, dated Mar. 14, 1989, have what is referred to as a protective group that is attached to the polymer. The protective group present in these materials is employed in the synthesis process to facilitate the formation of the resist material. In certain instances, such protective groups alter the solubility characteristics of the polymer.
Certain protective groups, when attached to the polymer, function as moieties that render the polymer relatively insoluble in alkaline solution. In lithographic processes, these moieties are removed upon irradiation and baking of the polymer film in the presence of a radiation-induced acid, and the polymer then becomes relatively more soluble in alkaline solutions. For example, in a synthesized polymer such as poly(4-tert-butoxycarbonyloxystyrene) the tert-butoxycarbonyl protective groups are removed and replaced by hydrogen to yield hydroxyl substituents. For convenience, "tert" will be shortened to "t" hereinafter. After a substantial percentage of the moleties (the t-butoxycarbonyl or other groups) have been cleaved from the exposed polymer, the polymer in the exposed region of the film is substantially more soluble in an aqueous alkaline developing solution.
The moieties are not cleaved from polymer in the unexposed regions. Therefore, the resist material in those regions is not as soluble in an alkaline solution. If an alkaline solution is used to develop the image projected onto the resist, the material in the exposed regions is dissolved by the developer solution while the material in the unexposed regions is not. It is by this mechanism that a positive tone image is developed that corresponds to the image projected into the resist material. Conversely, if the resist polymer in the unexposed regions is more soluble in the developer solution, then a negative tone image will be developed into the resist material.
If light is used as the energy source in a lithographic process, the process is referred to as photolithography. If such photolithographic processes utilize an exposure that occurs simultaneously over an entire device or a number of devices being processed on a substrate, the process utilizes what is considered a blanket exposure. A material, i.e., a resist, which is sensitive to the exposing radiation is coated onto a substrate, e.g., a silicon substrate, on which a plurality of devices will be formed. The coating material is (if desired) pre-exposure baked and is subjected to spatially discrete radiation, e.g., light that has been passed through a mask so that the light reaching the resist defines a discrete area. The discrete area defines a pattern that is to be transferred onto the underlying substrate either by negative or positive tone. The coated substrate is (if desired) post-exposure baked prior to image development of the discrete area on the substrate. The resists used in photolithography are referred to as "photoresists".
A blanket exposure is advantageous because it is relatively fast compared to other methods such as the raster scan technique that is employed when the energy used to expose the resist is a beam of electrons. However, generally, the resolution that is achieved through a blanket exposure with near ultraviolet or visible light is somewhat poorer than that achieved with other methods such as electron beam lithography.
Improved resolution with a blanket exposure can be achieved by using shorter wavelength light such as deep ultraviolet or X-ray light. One approach to a photoresist sensitive to shorter wavelength radiation employs a photo-acid generator (PAG) that produces an acid moiety upon irradiation with deep ultraviolet light, together with a polymer that reacts under the influence of heat with the generated acid. In another approach, the polymer itself provides an acid functionality upon chain scission, thus eliminating the need for an added PAG.
As stated previously, protective groups are cleaved from the polymer in the presence of acid. In some lithographic processes, acid is provided by a PAG. Typical PAG/acid sensitive polymer combinations include an onium salt as the PAG and a polymer such as poly(4-t-butoxycarbonyloxystyrene) that has a reactive substituent, e.g., a t-butoxycarbonyl protective group. (See Ito, et al. U.S. Pat. No. 4,491,628dated Jan. 1, 1985.) Typical acid sensitive polymers that provide an acidic functionality upon radiation-induced chain scission include poly(4-t-butoxycarbonyloxystyrene-sulfone). Such polymers are disclosed in U.S. Pat. No. 5,066,566 to Novembre. Such systems are generally referred to as chemical amplification systems since the production of one molecule of acid by radiation (e.g., light) induces a reaction in a plurality of reactive substituents in the acid sensitive polymer. Since protective groups are not cleaved from the resist polymer in the unexposed regions, it follows that acid is preferably not generated or otherwise present in the unexposed regions.
Attempts have been made to improve the sensitivity and to reduce the film shrinkage of chemically amplified resists. By improving the sensitivity of resists, less energy is required to create the image throughout the resist layer. In this regard, resist materials that have been partially deprotected (by the process disclosed in Canadian Patent Application 2,001,384, for example), have demonstrated enhanced sensitivity. Partial deprotection means that some, but not all, of the protective groups are cleaved from the polymer prior to the polymer's use in a lithographic process.
The '384 application states that some of the protective groups can be removed from the polymer by adding a strong acid such as sulfuric acid to the polymer which is in a heated solution. The acid is neutralized by the addition of base. The polymer is then isolated. The polymer is then combined with a PAG in a spinning solvent. This solution of polymer and PAG is then employed to spin coat a substrate which is subsequently exposed.
This process requires additional reactions after the acid-catalyzed deprotection to ensure the removal of added acid and base. A meticulous removal of the added acid is required before the resist is coated onto the substrate because any acid left in the polymer will degrade the polymer film upon post exposure baking. The added acid is neutralized by adding base. However, the base must be added carefully because any remaining base will neutralize a proportionate amount of photogenerated acid. The excess base must also be removed meticulously.
Although chemically amplified resists show great promise for fine line resolution, these materials have demonstrated a tendency to shrink after the exposure and post-exposure baking steps of the lithographic process. Shrinkage occurs when the protected polymer is heated in the presence of acid, which releases the protecting groups in the form of gaseous products such as CO.sub.2 and isobutylene from the polymer. Such shrinkage produces a loss of image quality and, in pan, counteracts resolution improvement available through use of deep ultraviolet, X-ray, or electron beam exposure. In device fabrication, because of particularly fine design rules, this film shrinkage can significantly and adversely affect the quality of the features produced in the lithographic process. Thus, although chemically amplified resists are extremely promising, some improvement is desirable.